Cycling snobs beware: I ride with a helmet mirror. Yes, I know they look dorky, but I have finally reached that point in life where I value my life more than I fear the disdain of bike snobs. I’ve purchased several different brands of helmet mirrors, but my favorite is the CycleAware Reflex Flexible Helmet Mirror.
This mirror mounts securely to your helmet with double-sided tape (included with the mirror). I own four cycling helmets and I use this mirror on three of them. There is no way to mount a mirror on my winter cycling helmet since my snow goggles have to be slide up on top of the helmet when not in use. My oldest helmet is four years old and is about to be replaced, but the CycleAware Reflex Flexible Helmet Mirror has hung on for every mile, both on the road and on the trails.
When riding into a headwind of over 30 MPH you might see the mirror vibrating a bit, but other than that it is very stable. CycleAware claims the mirror is removable, but I have never even tried. It is much easier to just buy a mirror for every helmet you own. The flexible shaft between the mount and the mirror itself is very easy to adjust. The large oval shape of the mirror provides a great field of view with a fantastic depth of field.